Superconducting switch having high current capability and high blocking resistance



June 6. 1967 p, MUELLER I 3,324,436

SUPERCONDUCTING SWITCH HAVING HIGH CURRENT I CAPABILITY AND HIGH BLOCKING RESISTANCE Filed Sept. 28, 1964 INVENTOR.

p /llp M M06416 fmwm United States Patent SUPERCONDUCTING SWITCH HAVING HIGH CURRENT CAPABILITY AND HIGH BLQCK- ING RESISTANCE Philip M. Mueller, Solon, Ohio, assignor to Lear Siegler, Inc, Los Angeles, Calif., a corporation of Delaware Filed Sept. 28, 1964, Ser. No. 399,782 7 Claims. (Cl. 338-32) This invention relates to a cryogenic switch and more particularly to cryogenic switches of the type that are turned 'on and olf by magnetic fields.

'-'It has been proposed to make cryogenic switches of this general character but their use has been limited to environments in which they are not required to carry heavy currents or to operate at substantial voltages or relatively high voltages. The use of cryongenic switches therefore has generally been confined to situations such as computers where the currents and voltages are very small and it is simply necessary to be able to detect the difference between the zero resistance of the switch when it is in the super-conducting state and the rather small resistance (i.e., a few ohms) when it is in the normal conducting state. Thus the usage of such switches has been severely limited. I

A general object of the present invention is to provide a cryogenic or super-conducting switch that will have very large resistance, e.g., of the order of several megohms, when in the normal state and essentially no resistance and a large current conducting capacity when in the super conducting state. Such a'switch, for example, may have the capacity to carry 100 amperes or more in the superconducting state and yet have such high resistance in the normal state that it can be used to control several hundred volts with no substantial flow of current when it is in the normal or off state. According to the present invention I accomplish this desired result by utilizing, instead of the superconductive metals that have beenemployed hitherto in super-conducting switches, super-conducting compounds made up of elements of which one of the elements is an insulator. Examples of thees materials are molybdenum boride, niobium boride, niobium nitride, niobium hydride, molybdenum nitride and zirconium nitride. These materials, in the normal state, have a resistivity of the order of 10,000 ohm centimeters. On the other hand they are capable of carrying current densities of the order of 75 to 80 thousand amperes per square centimeter. Thus in the normal state their resistivity is very high, approaching that of recognized insulators, while in the super-conductive state their conductivity is infinite. The present invention utilizes these properties in the construction of switches that have the advantages of previously known cryogenic switches in thatv they can conduct current in either direction, embodying no moving parts and can be turned on and off very. rapidly with the further important advantage that they have the ability to handle relatively large amounts of power at high voltages with very small leakage current in the turned-off state.

A preferred form of switch embodying the present invention is illustrated in the drawings in which:

FIGURE 1 somewhat diagrammatically illustrates a complete cryogenic switch embodying the invention; and

FIGURE 2 is a section taken as indicated on line 2-2 of FIGURE 1.

As shown in FIGURE 1 a preferred form of switch embodying my invention may comprise a cryogenic vessel indicated diagrammatically at 10. The vessel may be of conventional or known construction having an inlet 11 and a vent 12 which, if desired, may be connected to a vacuum pump. The vessel is filled with liquid helium and the 3,324,436 Patented June 6, 1967 elements of the super-conducting switch are immersed in the helium to maintain the elements of the switch below the critical temperature of the material employed, i.e., at a temperature at which they will remain super-conducting in the absence of an imposed external magnetic field.

The switch comprises a plate 14 composed of insulating material such as aluminum oxide which supports a thin layer 15 of a super-conducting material having the desired characteristics of large current-carrying capacity while in the super-conducting state and high resistivity while in the normal non-super-conducting state. Leads 16 and 17, also preferably composed of super-conducting wire are used to connect the super-conducting layer 15 to an external circuit. The leads may be composed of a known super-conducting metal or alloy. The super-conducting wires are joined as shown in the drawing to heavier copper conductors 18 and 19 within the cryogenic vessel. These conductors then are employed to connect the switch into the external circuit.

As is well known, super-conductive materials can be switched from a super-conductive condition to a resistive condition by means of an externally applied magnetic field. In order to control the switch of the present invention a coil 21 is shown as disposed within the cryogenic vessel 10 adjacent the super-conducting layer 15, the axis of the coil 21 being perpendicular to the plane of the layer 15. Leads 22 and 23 extend from the coil to the exterior of the cryogenic vessel 10; these may be connected to any appropriate control device, such as the simple circuit illustrated which includes a source of 24 and a switching device 25. A mechanical switch may be employed, or a transistorized control or any other convenient control may be used to control the energization of the coil. If desired, the coil 21 can be made of super-conducting wire. In any event, the thin layer 15 makes it possible to use a very small magnetic field because of the fact that such a layer has a very sharp anisotropic eiiect in relation to the critical magnetic field. Thus, the current flowing through the coil 12 can be very smallof the order of milliamperes for a coil having 10 turns, and hence the flow of current through the coil can be controlled very easily and high speed switching of the much greater current flowing through the layer can readily be obtained. Thus the particular configuration of the apparatus is advantageous in that the layer, although capable of carrying a large current, is driven to its normal state where it ,has a very high resistance by the application of a very small magnetic field. If desired, the coil 21 can be dis posed outside of the vessel 10.

When the coil 21 is energized the super-conducting layer is driven to the normal state, thus creating a resistance of the order of 5 megohms in the circuit. As soon as the coil is deenergized the layer becomes super-conducting again allowing current to flow through it with zero voltage drop. Since there is no resistance in the layer and since the leads 16 and 17 preferably are also composed of super-conducting material, there are no losses in the switch when it is turned on. When the switch is turned ofl, the fiow of current through it is so low that the PR losses are very small indeed; for this reason the necessity for replenishing the helium within the cryogenic vessel 10 arises largely from heat absorbed from the exterior of the vessel. The switch is further advantageous in that it can be turned off by very small control currents, regardless of the amount of current flowing through the switch and when the switch is on it can conduct in both directions. In this respect, as well as in the low current flow when the switch is turned off, the device has important advantages over conventional solid state devices such as silicon-controlled rectifiers. Also, the switch has a very low capacitance, because of its very simple configuration.

As noted above, the layer 15 is composed of a superconducting compound made up of elements of which one is an insulator. Suitable materials with their critical temperatures, all of which are above the boiling point of helium at normal atmospheric pressure are given in the list below:

MOB 4.0

NbB 6.0 NbH 7-13 MON l2 NbN 15-16 ZrN 9.3

With these materials, the super-conducting layer can be in the form of a film or foil having a thickness of about 0.001 inch (0.04 mm.), a width of about /2 inch (12.5 mm.) and a length of about 1 inch (25 mm.). A layer of such dimensions composed of any of the materials specified above would have a resistance of the order of megohms in the normal state. Thus, even with a voltage of as high as 1,000 volts, the current flow through the layer in the normal state would be negligible. On the other hand, a layer of these dimensions can carry a current of 100 amperes and still be well within the limits of critical current density.

While a thin layer of the type shown in the drawings is prefererd, it'is possible to utilize a super-conducting wire composed of one of the above listed materials as the switch. A wire having the same cross-sectional area as the layer described above would have about the same resistivity in the normal state and about the same current carrying capacity in the super-conducting state as the layer. However, the wire is less advantageous from the standpoint of control andwould require magnetic field of greater strength to drive the wire to the normal state.

As noted above, the leads 16 and 17 are preferably composed of a super-conducting material to avoid losses during the time that the switch is on. Similarly the coil 21 and its leads 22 and 23 may also be composed of superconducting material and in some instances it may bedesired to turn the switch off by causing a current to flow in the coil 21 and letting the current flow persist due to the super-conductivity of the metal until it is desired to turn the switch on. This can be accomplished in a known manner and requires a minimum amount of energy and the'absorption of a minimum amount of heat by the liquid helium surrounding the coil.

From the foregoing description it will be evident that the switch of my invention provides a convenient method of controlling rather large amounts of current flowing in either direction. The switch can be turned on and 01f almost instantaneously, will remain in one state indefinitely until changed to the other, has very high resistance and conducts very little current when in the ofi? state and inserts essentially no resistance or capacitance into the circuit when it is in the on state. Thus the switch is exrernely useful where it is desired to rapidly control fairly large amounts of power regardless of the direction of the current, a result that cannot be obtained With presently known solid state devices.

Those'skilled in the art will appreciate that various changes and modifications can be made in my invention without departing from the spirit and scope thereof. The

essential characteristics of the invention are defined in the appended claims.

I claim:

1. A super-conducting switch comprising a conductor consisting of a material that is super-conducting at temperatures below its critical temperature and that has a resistivity of the order of 10,000 ohm centimeters in the normal state at temperatures above its critical temperature, leads for connecting the conductor to an external circuit, means for mainaining the conductor at a temperature below its critical temperature and means for impressing a magnetic field on the conductor to cause it to assume the normal state at temperaures below is critical temperature.

2. A super-conducting switch comprising a thin layer consisting of a material that is super-conducting at temperatures below its critical temperature and that has a resistivity of the onder of 10,000 ohm centimeters in the normal state, leads for connecting the ends of said layer to an external circuit, the resistance between the leads when the material is in the normal state being of the order of several megohms and being substantially zero when the material is in the super-conducting state, means for maintaining the layer at a temperature below its critical temperature, a coil for impressing a magnetic field on the layer, and means for energizing the coil to cause the layer to be restored to the normal state while remaining below its critical temperature.

3. A super-conducting switch according to claim 2 wherein the layer is substantially flat and the axis of the coil is normal to the plane of the layer.

4. A super-conducting switch made according to claim 2 wherein the layer comprises a material from the group consisting of molybdenum boride, niobium boride, niobium nitride, niobium hydride, molybdenum nitride and zirconium nitride.

5. A super-conducting switch according to claim 2 wherein the super-conducting layer is disposed within a cryogenic vessel and the coil is composed of a superconducting material and is disposed within the cryogenic vessel.

6. A super-conducting switch according to claim 2 wherein the super-conducting layer is disposed within a cryogenic vessel and wherein the lead-in wires for conducting current to the super-conducting layer are also composed at least in part of super-conducting material.

7. A super-conducting switch according to claim 2 wherein the super-conducting layer is composed of one of the materials from the group consisting of molybdenum boride, niobium boride, niobium nitride, niobium hydride, molybdenum nitride and zirconium nitride, the layer having a thickness of the order of 0.04 mm., a width of the order of 12.5 mm. and a finite length greater than its Width.

References Cited UNITED STATES PATENTS 2,666,884 1/1954 Ericsson et al 338-32 3,048,707 8/1962 Nyberg a 338-32 3,187,236 6/1965 Leslie 174-158 3,239,787 3/1966 Reeber 338-32 3,259,866 7/1966 'Miles et al. -Q 338-32 RICHARD M. WOOD, Primary Examiner.

W. D. BROOKS, Assistant Examiner. 

2. A SUPER-CONDUCTING SWITCH COMPRISING A THIN LAYER CONSISTING OF A MATERIAL THAT IS SUPER-CONDUCTING AT TEMPERATURES BELOW ITS CRITICAL TEMPERATURE AND THAT HAS A RESISTIVITY OF THE ORDER OF 10,000 OHM CENTIMETERS IN THE NORMAL STATE, LEADS FOR CONNECTING THE ENDS OF SAID LAYER TO AN EXTERNAL CIRCUIT, THE RESISTANCE BETWEEN THE LEADS WHEN THE MATERIAL IS IN THE NORMAL STATE BEING OF THE ORDER OF SEVERAL MEGOHMS AND BEING SUBSTANTIALLY ZERO WHEN THE MATERIAL IS IN THE SUPER-CONDUCTING STATE, MEANS FOR MAINTAINING THE LAYER AT A TEMPERATURE BELOW ITS CRITICAL TEMPERATURE, A COIL FOR IMPRESSING A MAGNETIC FIELD ON THE LAYER, AND MEANS FOR ENERGIZING THE COIL TO CAUSE THE LAYER TO BE RESTORED TO THE NORMAL STATE WHILE REMAINING BELOW ITS CRITICAL TEMPERATURE. 